Re: How do you know how much power to use in sparring?
Honestly, I think sparring is one step above slap boxing. When you're new, you'll be nervous and you'll want to hurt people with power, but I think the purpose of sparring is to work on the sweet science.
Countering, defense, footwork.
I've been to gyms though where people just beat eachother up, but I don't think that's how it should be. Save that for the actual figth.
It's easy to punch fast and light.
Just pull your punches at the last moment. Don't put full impact into them.
I used to have the same problem as you, but it's not that difficult. The reason you can't do it is because you're probably too nervous and tense.